Method of manufacturing micromechanical devices

ABSTRACT

Micromechanical devices are formed on a substrate using a sacrificial layer deep X-ray lithography process to produce a rotating microrotor which is driven magnetically. The rotor typically has a diameter of a few hundred microns or less and is formed as a free structure which is assembled onto a hub formed on a substrate. Stator pole pieces are formed on the substrate of a ferromagnetic material surrounding the rotor, and are al 
     This invention was made with U.S. government support awarded by the National Science Foundationn (NSF), Grant #EET-88-15285. The U.S. government has certain rights in this invention.

This invention was made with U.S. government support awarded by theNational Science Foundationn (NSF), Grant #EET-88-15285. The U.S.government has certain rights in this invention.

FIELD OF THE INVENTION

This invention pertains generally to the field of semiconductor andmicromechanical devices and processing techniques therefor, andparticularly to microminiature magnetic devices.

BACKGROUND OF THE INVENTION

Deep X-ray lithography involves a substrate which is covered by thickphotoresist, typically severally hundred microns in thickness, which isexposed through a mask by X-rays. X-ray photons are much more energeticthan optical photons, which makes complete exposure of thick photoresistfilms feasible and practical. Furthermore, since X-ray photons are shortwavelength particles, diffraction effects which typically limit devicedimensions to two or three wavelengths of the exposing radiation areabsent for mask dimensions above 0.1 micron. If one adds to this thefact that X-ray photons are absorbed by atomic processes, standing waveproblems, which typically limit exposures of thick photoresist byoptical means, become an non-issue for X-ray exposures. The use of asynchrotron for the X-ray source yields high flux densities--severalwatts per square centimeter--combined with excellent collimation toproduce thick photoresist exposures without any horizontal run-out.Locally exposed patterns should therefore produce vertical photoresistwalls if a developing system with very high selectivity between exposedand unexposed photoresist is available. This requirement is satisfiedfor polymethylmethacrylate (PMMA) as the X-ray photoresist and anaqueous developing system. See, H. Guckel, et. al., "Deep X-Ray and UVLithographies For Micromechanics", Technical Digest, Solid State Sensorand Actuator Workshop, Hilton Head, SC, June 4-7, 1990, pp. 118-122.

Deep X-ray lithography may be combined with electroplating to form highaspect ratio structures. This requires that the substrate be furnishedwith a suitable plating base prior to photoresist application. Typicallythis involves a sputtered film of adhesive metal such as chromium ortitanium which is followed by a thin film of the metal which is to beplated. Exposure through a suitable mask and development are followed byelectroplating. This results, after cleanup, in fully attached metalstructures with very high aspect ratios. Such structures were firstreported by W. Ehrfeld and co-workers at the Institute for NuclearPhysics at the University of Karlsruhe in West Germany. Ehrfeld termedthe process "LIGA" based on the first letters of the German words forlithography and electro-plating. A general review of the LIGA process isgiven in the article by W. Ehrfeld, et. al., "LIGA Process: SensorConstruction Techniques Via X-Ray Lithography", Technical Digest, IEEESolid-State Sensor and Actuator Workshop, 1988, pp. 1-4.

A difficulty with the original LIGA process is that it can only producefully attached metal structures. This restricts the possible applicationareas severely and unnecessarily.

The addition of a sacrificial layer to the LIGA process facilitates thefabrication of fully attached, partially attached, or completely freemetal structures. Because device thicknesses are typically larger than10 microns and smaller than 300 microns, freestanding structures willnot distort geometrically if reasonable strain control for the platedfilm is achieved. This fact makes assembly in micromechanics possibleand thereby leads to nearly arbitrary three-dimensional structures. SeeH. Guckel, et. al., "Fabrication of Assembled Micromechanical Componentsvia Deep X-Ray Lithography," Proceedings of IEEE Micro ElectroMechanical Systems, Jan. 30 -Feb. 2, 1991, pp. 74-79.

SUMMARY OF THE INVENTION

In accordance with the present invention, micromechanical devices areformed on a substrate utilizing a sacrificial layer deep X-raylithography process to produce a rotating micropart or rotor which isdriven magnetically. The rotor, which typically will have a diameter ofa few hundred microns or less, is formed as a free structure which isassembled onto a hub formed on the substrate. The rotor may be formed tohave low-reluctance and high-reluctance paths through it. Surroundingthe rotor on the substrate are stator pole pieces which are structuredto channel magnetic flux to the rotor. Both the rotor and the statorpole pieces are formed of a ferromagnetic material, such as nickel,which is well suited to providing a magnetic flux path.

The stator pole pieces form part of a means for providing a rotatingmagnetic field in the region of the rotor. The rotating magnetic fieldcan be provided, for example, by a rotating magnet beneath the substrateon which the rotor is mounted, or by an external magnet which couples amagnetic flux to the stator pole pieces in a time varying manner tocreate the rotating magnetic field. In a preferred embodiment, theelectrical excitation to provide the magnetic field in the pole piecesis provided by conductors formed integrally on the substrate with thestator pole pieces. Such conductors may be formed as conductive stripssurrounding upstanding stator pole pieces or as adjacent conductorsformed on the substrate which provide a current which encircles the polepiece to induce a magnetic flux therein. By providing perpendicularpairs of pole pieces, each excited by sinusoidal currents, 90° out ofphase with the other, rotating magnetic fields can be generated in therotor region which will drive the rotor to rotate with the magneticfield.

Utilization of a sacrificial layer in conjunction with the LIGA processallows the rotor to be formed of a ferromagnetic metal such as nickel atsubstantial vertical dimensions, e.g., a hundred microns or more, whichallows formation of stable structures which can be handled and assembledinto place. Additional gears can also be formed by the sacrificial layerprocess which can be engaged to the rotor, such as by having gear teethon the periphery of the rotor engage the gear teeth of the gears. Thegears can thereby be utilized to provide power transfer to othermechanically driven devices.

Further objects, features and advantages of the invention will beapparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a simplified cross-sectional view of an initial processingstep in accordance with the present invention wherein a sacrificiallayer is formed on a substrate.

FIG. 2 shows the structure of FIG. 1 with the addition of a platingbase.

FIG. 3 shows the structure of FIG. 2 with the addition of a layer ofPMMA photoresist.

FIG. 4 shows the structure of FIG. 3 being exposed to X-rays through anX-ray mask.

FIG. 5 shows the structure of FIG. 4 after and electroplating of nickelinto the area of the developed PMMA.

FIG. 6 shows the structure of FIG. 5 after removal of the PMMA.

FIG. 7 is a perspective view of the remaining nickel structure on thesubstrate after removal of the PMMA and the sacrificial layer.

FIG. 8 is a simplified perspective view of a micromechanical devicehaving a rotating rotor which is driven by a magnet rotating beneath thesubstrate on which the rotor is mounted.

FIG. 9 is a plan view of a micromechanical motor in accordance with thepresent invention formed on a substrate.

FIG. 10 is a photomicrograph of a micromechanical motor of the typeshown in FIG. 9.

FIG. 11 is a perspective view of an exemplary magnetic field generatorwhich can be coupled to the micromotor of FIG. 9 to drive the rotor ofthe same.

FIG. 12 is a cross-sectional view of a first step in a process for theformation of a magnetic micromotor.

FIG. 13 is a plan view of the structure shown in FIG. 12.

FIG. 14 is a cross-sectional view of the structure of FIG. 12 afterfurther step of processing.

FIG. 15 is a plan view of the structure of FIG. 14.

FIG. 16 is a cross-sectional view of the structure of FIG. 14 afterfurther processing.

FIG. 17 is a plan view of the structure of FIG. 16.

FIG. 18 is a cross-sectional view of the structure of FIG. 16 afterassembly, of a rotor onto the structure.

FIG. 19 is a plan view of the structure of FIG. 18.

FIG. 20 is a cross-sectional view through a substrate illustrating afirst step in another process for forming a magnetic micromotor.

FIG. 21 is a plan view of the structure of FIG. 20.

FIG. 22 is a cross-sectional view of the structure of FIG. 20 after afurther step of processing.

FIG. 23 is the plan view of the structure of FIG. 22.

FIG. 24 is a cross-sectional view of the structure of FIG. 22 afterfurther processing.

FIG. 25 is a plan view of the structure of FIG. 24.

FIG. 26 is a cross-sectional view of the structure of FIG. 24 afterfurther processing.

FIG. 27 is a plan view of the structure of FIG. 26.

FIG. 28 is a cross-sectional view of the structure of FIG. 26 afterfurther processing.

FIG. 29 is a plan view of the structure of FIG. 28.

FIG. 30 is a cross-sectional view of the structure of FIG. 28 afterfurther processing.

FIG. 31 is a plan view of the structure of FIG. 30.

FIG. 32 is a cross-sectional view of the structure of FIG. 30 afterfurther processing.

FIG. 33 is a plan view of the structure of FIG. 32.

FIG. 34 is a simplified perspective view, partially cut away, of thecompleted magnetic micromotor of the type illustrated in FIGS. 32 and33.

FIG. 35 is a photomicrograph of another rotor and stator structure inaccordance with the inventors.

FIG. 36 is a photomicrograph of an assembled stepping motor inaccordance with the invention.

FIG. 37 is a photomicrograph of an assembled wobble motor in accordancewith the invention.

DETAILED DESCRIPTION OF THE INVENTION

The sacrificial layer LIGA process utilized in the present invention isillustrated in FIGS. 1-7 and is described below with reference to thesefigures.

LIGA processing involves maximum temperatures of 180° C. or so. Thisproperty makes material selection for the sacrificial layer or layersquite easy because many material candidates exist. The requirements aresimply that the sacrificial material can be applied with good adhesion,that it can be locally defined, and that it can be covered with aplating base without flank coverage problems. In addition, X-ray damagemust be minimized and removal via surface micromachining or lateraletching must not modify the plated metal. Reasonable systems for platednickel are polysilicon with hydrazine as an etch or various double metalcombinations. A very convenient sacrificial layer is a spin coatedpolyimide film which remains soluble in bases for bake-out temperaturesto 270° C. or so. This film can be covered by a positive photoresistwhich enables pattern transfer from an optical mask to the photoresist.Since the developer for positive resist is basic, pattern transfer tothe polyimide is achieved via dissolution in the exposed section of thesubstrate. A heat cycle follows which renders the polyimide mildlysoluble in bases and therefore can be followed by a second or thirdsacrificial layer with a different polyimide thickness.

The patterned sacrificial layer, shown at 40 in FIG. 1 on the topsurface of a substrate 41, is covered by the plating base 43 as shown inFIG. 2. Where nickel is to be plated, a satisfactory plating base is asputtered film which consists of 150Å of titanium followed by 150Å ofnickel. This film cannot be patterned in normal situations becauseelectronic contact to the plating fluid has to be established. Theprepared substrate is next subjected to the basic LIGA procedure: thickPMMA photoresist application to form a layer 45 as shown in FIG. 3,exposure through an X-ray mask 46 as shown in FIG. 4, developing of thePMMA and electroplating to form the nickel part 48 as shown in FIG. 5.However, there is a severe technical problem: the X-ray mask isgeometrically related to the patterned sacrificial layer and thereforemust be aligned to the processed substrate. The alignment requirement iscomplicated by the fact that the exposure source is physically separatefrom the alignment tools. In a preferred procedure, alignment isfollowed by the use of a clamping mechanism which can be removed fromthe alignment tool and allows transport and insertion into the X-raysource without mask to substrate slippage. The actual alignment can beaccomplished by using optical techniques in a double-sided mask alignerwhich can accommodate the alignment-clamping fixture. Optical alignmentbecomes less complicated if the X-ray mask is totally or locallyoptically transparent. This can be accomplished more directly if theX-ray mask blank is formed from strain controlled silicon nitride. Theabsorber is a patterned layer of gold which is supplied by additiveprocessing, e.g., by electroplating, and therefore has the desired edgeacuity which is difficult to obtain with subtractive processing, e.g.,etching, for absorber thicknesses near 4 microns. Mask blanks made fromother materials, such as fine grained polysilicon, can also be used butmust have locally transparent regions for the present alignment schemewhich leads to more complicated mask fabrication procedures.

The processing continues with the removal of the used PMMA to leave theisolated structure 48 as shown in cross-section in FIG. 6. This can beaccomplished by re-exposure and a second developing cycle or by suitableplasma etching. The unwanted sections of the plating base are removed bywet etching and the exposed sacrificial layer is dissolved in ammoniumhydroxide to form the finished structure. As illustrated in FIG. 7, thisstructure has a cavity 50 where the sacrificial layer 40 was located,and allows the structure 48 to be completely removed from the substrate41 if the sacrificial layer is formed under the entire structure.

The sacrificial layer LIGA process can be applied to create a variety ofmicrostructures formed of metal which have previously been formed ofsilicon via silicon micromachining. Generally, the sacrificial layerLIGA process allows simpler processing with typical mask levelreductions from six or eight to two, and very high aspect ratiostructures which scale directly into higher force outputs for actuators.Moreover the process can be used with any planar substrate to which thesacrificial layer and the plating base can be attached. Therefore,typical silicon substrates are acceptable, but so are quartz, sapphire,glass, plastic, and metal substrates.

Nickel and several of its alloys are ferromagnetic. They exhibit highpermeabilities (low reluctance) for low flux densities and saturate athigh magnetic fields of 10,000 gauss or so. At 3 micron working gaps and1×10⁵ volt/cm electric fields, magnetic fields of 10,000 gauss produceroughly three orders of magnitude larger gap pressures than can beobtained with electrostatic fields.

Magnetic fields are established by currents. The driving point impedancefor single turn excitation conductors typically involves one or two ohmsof DC resistance and inductance in the few nanohenry range. Currentdrive to 200 milliamperes will therefore involve voltage drops below afull volt. Bipolar rather than CMOS circuitry can achieve this easilyand is nearly a perfect match for micromagnetics.

An exemplary magnetically actuated micro-device is illustrated in FIG.8. A permanent magnet 55 mounted for rotation on the shaft 58 of a motor58 is located beneath the backside of a substrate 59 and its field iscoupled through stator poles 61 to a rotating rotor 62 on the substratesurface. Rotation of the magnet 55 beneath the stator, or equivalently,two phase excitation by an electromagnet of the stator, produces arotating magnetic field which drives rotation of the rotor 62. Thisfield can be increased by bringing the magnetic excitation closer to theactuator. Most importantly, the minimum magnetic field which causesrotation can be measured by a suitable gauss meter. Such 100 micronstructures are found to rotate quite satisfactorily at magnetic fieldstrengths of 6 gauss or so. This is only about one order of magnitudeabove the earth's magnetic field.

A basic magnetic micro-motor 98 is shown in plan view in FIG. 9, andFIG. 10 is a photomicrograph of the completed structure. The micromotor98 is formed on a substrate 99 and has a rotor 100 rotatable about apost 10 which acts as the hub for the rotor. The motor includes two setsof stator pole pieces 102 and 103 arranged perpendicular to each otherto allow two phase driving of the rotor 100. The rotor 100 has aperipheral ring with an outer surface with gear teeth 104 thereon whichmesh with the teeth of a gear 105, and rotates about a shaft 106. A gear108 rotating about a shaft 109 is engaged to the gear 105, and a gear110 rotating about a shaft 111 is engaged to the gear 108. Power maythus be taken off the train of gears 105, 108, and 110 by othermechanical devices (not shown) to accomplish desired mechanical tasks.

In addition to being driven by an external rotating magnet, the rotor100 can also be driven by applying a time varying magnetic field betweenthe stator pole pieces 102 and 103. An example of an external devicewhich can be utilized to couple a time varying magnetic flux to thestator pole pieces 102 and 103 is shown in FIG. 11. This structureutilizes two magnetic metal core pieces for providing a quadrature fluxpaths. The first core piece includes a U-shaped bar 115 of ferromagneticmaterial such as nickel which is engaged to quarter pieces 116 and 117,and a second U-shaped magnetic bar 120 which is engaged to quarterpieces 121 and 122. A first phase winding 124 is wound around the bar115 and a second phase winding 126 is wound around the bar 120.Extending from the bottom of the quarter pieces 116, 117, 121 and 122are four elongated quarter pieces 129-133 each of which is joined to thebottom of one of the quarter pieces 116, 117, 121 and 122. The bars andquarter pieces function as flux guides and are also formed offerromagnetic material such as nickel. Thus, when the first phasewinding 124 is provided with current, a magnetic flux path existsbetween the ends of the quarter pieces 129 and 131, and When a currentis passed through the second phase winding 126, a magnetic flux path isformed between the ends of the quarter pieces 130 and 132. The free endsof the quarter pieces 129-132 are adapted to engage with the top of thewidened portions of the four stator pole structures 102 and 103 so that,for example, a current passed through the first phase winding 124 willprovide a flux path between the stator pole pieces 102 through therotor, whereas current passed through the coil 126 will provide amagnetic flux path between the stator pole pieces 103 through the rotor100. By providing sinusoidally varying currents to the two coils 124 and126 that are 90° out of phase, a resultant magnetic field is generatedin the region of the rotor that rotates around the axis of the rotor. Itis noted that providing magnetic flux in this manner allows the rotor tobe driven without an electrical connection to the substrate 98 on whichthe rotor is mounted.

The rotor may be one which is permanently magnetized along its way (lowreluctance) axis. This will increase the maximum torque which can beproduced. In such a case, the rotor material should be preferably bemagnetically bard (i.e., have a square B-H loop) with high coercivity.This may be accomplished be electroplating an alloy of nickel with highcoercivity (e.g., nickel-cobalt) and by using approximate anneal cycles.The electroplated part may be magnetized during electroplating or afterrelease from the substrate in a sufficiently intense magnetic field.

Since ferromagnetic materials like nickel exhibit hysteresis, the rotormay be non-solvent (have no variation in reluctance along any directionof its axis) and have torque generated by the alternating or rotatinghysteresis losses which will occur when the rotor is subjected to arotating magnetic field.

FIG. 35 shows an alternative rotor construction in which the rotor hasmultiple poles formed as spokes or gear teeth, and thus has multiple lowand high reluctance paths therethrough. FIG. 36 shows another spoke-likerotor construction which can be operated as a stepping motor. FIG. 37shows a round rotor having an internal opening larger than the hub sothat the rotor rotates eccentrically and functions as a wobble rotor.

The construction of a motor in which the electrical excitation isintegrated on the substrate with the stator is shown in FIGS. 12-19.With reference to these figures, the construction can be brieflydescribed as follows. First, a plating base of titanium (about 150Angstroms) and nickel (about 150 Angstroms) is sputtered onto asubstrate 130, which may be of any relatively, flat, polished, cleanmaterial, such as single crystal silicon, quartz, sapphire, glass, orcomparable substrate materials. Linear PMMA is then spun onto theplating base 131 to a thickness of about 6 μm and the PMMA is thenpatterned optically (X-ray lithography is not necessary because of therelatively thin layer of PMMA which is being patterned) and nickel iselectroplated to about 5 μm. The PMMA and the plating base in the areasother than underneath the nickel is then removed. This leaves thestructure as shown in cross-section in FIG. 12, and in plan view in FIG.13, in which there is a ring shaped magnetic flux return path 133 formedof nickel and four nickel pads 134 and 135 for the two phases of thestator.

The polyimide isolation layer is then spun on to roughly 3 μm thicknessand soft annealed. The isolation layer is then patterned to open upstator anchors and contacts to current carrying lines, and a platingbase of titanium and nickel is described as before is then sputteredonto the overall structure. The structure at this point is illustratedin cross-section in FIG. 14, and in elevation in FIG. 15, in which thepolyimide isolation layer 138 overlays the other structures except foropenings 140 for stator contacts and openings 141 for conductive wireconnections to the stator electrical conductors.

PMMA is then cast to the desired thickness of the major structures,e.g., 100 microns or more, and the X-ray mask is then aligned to thePMMA which defines the stator, hub and winding leads. Synchrotron X-rayexposure is then carried out, with developing of the PMMA to leave thedesired cavities, and then nickel is electroplated to the desiredthickness. The remaining PMMA is then removed, leaving structures asillustrated in cross-section in FIG. 16 and in plan view in FIG. 17.These include stator pole pieces 150 for a first phase and pole pieces152 for a second phase, formed of the electroplated nickel, which aremounted to the substrate by contact with the plating base at the exposedportions of the magnetic flux return ring 133, and a central hub 153.Both of these structures are relatively high, e.g., in the range of a100 μm. Note that the polyimide release layer 138 is left extendingoutwardly from the stator pole pieces 150 and 152 to an inner peripheralwall 156. Current contacts 154 and 155 are formed of nickel for provinga current path connection to the conductors 134 and 135 which underliethe stator pieces 150 and 152.

The final step, as shown in FIGS. 18 and 19, is the addition of a rotor160 which fits over the hub 153 between the pole pieces 150 and 152 andwhich is supported at its edges for rotation by a portion of thepolyimide isolation layer 138. Illustrative electrical connections forthe phase windings include a first wire 164 for the first phase whichconnects to one of the pole conductors 154, with the circuit beingcompleted from the other end of the pole conductor through a conductor165 to the diametrically opposite pole conductor 154, and from the otherpole conductor through an electrical lead 166. The second phaseconnections are provided by a wire 168 connected to one of theconductors 155 for the second phase, from the other end of the conductor155 by a wire 169 to the diametrically opposite phase conductor 155, andthence completed from the other phase conductor through a wire 170. Byapplying sinusoidally varying electrical currents to the first phasewires 164, 166 and to the second phase wire 168, 170 which are 90° outof phase with one another, a rotating magnetic field is provided throughthe rotor to cause the rotor to be driven to follow the field.

The following describes the processing steps for carrying out theconstruction of the motor structure described above.

FIRST LAYER

Define inter-stator magnetic flux paths and returns for current carryingwires.

A plating base of 150 Angstroms of titanium followed by 150 Angstroms ofNickel is sputtered onto the substrate material. This initial stepdefines the first layer which provides the inter-stator magnetic fluxpaths and the returns for the current carrying wires.

2. Spin on linear PMMA (KTI PMMA 9% solids, dyed with coumarin 6 in theratio 25 ml PMMA to 50 mg coumarin 6) to a thickness of 6 μm using ananneal sequence of: 60° C./hr. ramped up to 180° C., hold for one hour,60° C./hr. ramp down to room temperature, for each layer of PMMA (4layers of PMMA are necessary at 1.5 μm each when spun at 2 krpm for 30seconds).

3. Pattern PMMA with a 2-layer photo resist process.

(a) Spin KTI 809 PR on PMMA at 5 krpm,

(b) Prebake at 90° C. for 15 minutes,

(c) Expose the layer of 809 with first layer mask and develop in 1:1 809developer:water,

(d) Blanket deep UV (230 nm) expose PMMA,

(e) Develop in the following 3 baths for:

Five minutes in:

60% vol. 2-(2-butoxy ethoxy) ethanol

20% vol. Tetrahydro-1-4 oxizin (morpholin(e)

5% vol. 2-Aminoethanol (ethanolamine)

15% vol. distilled, deionized water;

Twenty minutes in:

80% vol. 2-(2-butxoyethoxy) ethanol

20% vol. water.

Then five minutes in:

100% water. (All baths are held at 35.0° C.±or -0.5√ C).

4. Preparation for electro-plating:

(a) O₂ plasma descum for 2 minutes. (In Plasma Therm 1440, for example:O₂ =25 SCCM, pressure=30 mT,power=50 watts).

(b) Treat nickel plating base with 5% HCl for 15 minutes.

5. Electroplate nickel to 5 μm thickness at 50 mΛ/cm² in a nickelsulfamate plating bath.

6. Remove PMMA by blanket deep UV exposure and a development cycle as in3(e) above.

7. Remove plating base: Ni 40 minutes in 5% HCl, 5 minute water rinse;Ti, 2 minutes in 200:1 HF (4.9% HF), 15 minute water rinse.

SECOND LAYER

Define isolation layer contact areas.

8. Spin on PiRL(I) (polyimide release layer from Brewer Science, Rolla,Mo.) to a thickness of 3 μm and bake at 210° for one minute on a hotplate.

9. Pattern PiRL to open up plating areas for current wires and statorflux paths:

(a) spin Shipley 1400-27 PR on PiRL,

(b) align and pattern contacts to flux paths and current wires in1400-27 PR layer which carries over to PiRL layer (MF-321 Shipleydeveloper may be use(d),

(c) remove 1400-27 PR using acetone,

(d) descum with O₂ plasma for one minute as in 4(a) above.

(e) the PiRL at this point may be hard baked between 300° C. and 350° C.if it will not be removed at the end of the process (step 19 below).

10. Sputter 150 Angstroms Ti followed by 150 Angstroms Ni.

THIRD LAYER

Deep X-ray lithography defined layer.

11. Cast PMMA typically to about 100-200 μm.

12. Align x-ray mask containing wire definitions and stator definitionsto PiRL contact openings.

13. Synchrotron radiation expose the cast PMMA to a dose of 3.5 kJ/cm²at the bottom of the cast PMMA at 1 Gev beam energy (e.g., such asprovided by the Aladin Synchrotron, Stoughton, Wisconsin).

14. Develop using the same sequence as in 3(e) above wherein the timesare about 20 minutes in the first bath, 20 minutes in the second bath,and 5 minutes in the third bath.

15. Prepare the substrate for nickel plating:

(a) O₂ plasma descum as in 4(a) above in 15 seconds on/45 seconds offintervals for a total on-time of 2 minutes,

(b) 5% HCl cut for 15 minutes.

16. Nickel electroplate at 50 mΛ/cm² in nickel sulfamate bath (platingrate is about 1 μm/minute).

17. Remove cast PMMA via second synchrotron blanket X-ray exposure anddevelopment as in Step 14 above.

FOURTH LAYER

Assemble rotor on center hub, the rotor having been processed on aseparate substrate and released via an unpatterned PiRL sacrificiallayer.

18. Remove plating base:

Ni, 40 minutes in 5% HCl 5 minutes of water rinse,

Ti: 2 minutes in 200:1 HF(4.9%), 15 minutes of Water rinse.

19. (Optional): The PiRL layer may be removed with a suitable basicsolution (for example, in 3:1 H₂ O:NH₄ OH).

20. Bond out wiring to the individual stators (may be soldered or silverepoxied to nickel windings).

21. Assemble the corresponding rotor on the center post.

It is noted that the current windings could be plated from copper onboth the first and third layers by separating the stator and wirepatterns into individual masks, and repeating the first and third layerprocessing using copper (or gold, or any high conducting, non-magneticelectroplated metal). Using nickel for the windings will not result in asubstantive change in the performance of the device, although it doeshave higher resistivity than either copper or gold.

The formation of an alternative electrical excitation system for amagnetic micromotor is shown in FIGS. 20-33, and forms the motorstructure shown in perspective view in FIG. 34. The basic process can besummarized with reference to these figures, starting with FIGS. 20 and21. First, a plating base is sputtered onto a starting substrate, forexample, glass, and comprises a layer of titanium (about 150 Angstroms)and nickel (about 150 Angstroms) to provide a plating base 201 on thetop surface of the substrate 200. Linear PMMA is then spun on in a layerwith a thickness of about 6 μm. The PMMA is patterned optically andnickel is electroplated at about 5 μm to form the nickel base layer. ThePMMA and the plating base is then removed to leave a nickel flux returnpath 202 in a ring with a center opening 204.

With respect to FIGS. 22 and 23, a first polyimide (PiRL) layer is spunon roughly to 3 μm thickness and soft annealed to form the layer 208.Aluminum is then sputtered on to about 4 μm and windings are patternedwith a P-A-N etch to form the windings 210 which have a general U-shapedwith a center bridge section 211.

With reference to FIGS. 24 and 25, a second PiRL isolation layer 214 isthen spun on to about 3 microns and soft annealed.

As shown in FIGS. 26 and 27, vias are then etched in the PiRL for statoranchoring and rotary etch supports, and the remaining PiRL is hardannealed (at about 300° C.). This leaves openings 216 and 217 in thePiRL and an inner wall 219 Which surrounds the open area 204. Note thatthe conductive layer 211 is encased within the PiRL layers 208 and 214.

With reference to FIGS. 28 and 29, a third layer of PiRL is spun on forcontact protection in the areas 221 to about 2 μm and soft annealed. Theprotection layer is then patterned leaving the PiRL only over thecontacts in the areas 221 and a plating base 222 of titanium and nickelis then sputtered on.

With reference to FIGS. 30 and 31, PMMA is then cast onto the structure,an X-ray mask containing stator and hub definition is then aligned tothe structure and the synchrotron X-ray exposure is made. After removalof the PMMA in the exposed areas, nickel is electro-plated in and theentire remaining PMMA is exposed to synchrotron X-rays and developed toremove the PMMA, leaving the stator structures 223 in the central hub224.

With reference to FIGS. 32 and 33, the plating base is then removed, thePiRL contact protection layer is removed, and the contact wires 230 arethen bonded to the exposed areas of the conductors 210 to complete thecircuit. The rotor 235, constructed separately on another substrateusing the sacrificial layer technique shown in FIGS. 1-7, is thenassembled onto the hub 224. As is illustrated in FIG. 33, the rotorpreferably has a central bar section 236 which provides a low reluctancea flux path between the stator pole pieces 223 and a thinner peripheralring section 237 which, with the openings 236, defines a high reluctancepath perpendicular to the low reluctance path. The openings whichaccepts the hub is formed in the center bar. The fully assembled motoris also shown in simplified perspective view in FIG. 34.

The processing steps are described in more detail as follows:

FIRST LAYER

The magnetic flux path is patterned to provide the return path for thestator.

The first step corresponds to steps 1-7 in the integrated processingsequence described above.

SECOND LAYER

1. Pattern aluminum winding layer.

2. Spin-on PiRL to 3 μm and bake at 210° C. for one minute on a hotplate.

3. Sputter four μm of aluminum.

4. Pattern the aluminum using a standard wet-edge process, for example,use 1375 Shipley photoresist, align Winding layer to underlying nickelflux path layer, etch aluminum in standard phosphoric-acetic-nitric acidaluminum etch solution, and remove photoresist with commercial PRstripper, (e.g., Shipley 1165).

5. Spin-on second PiRL layer (3 μm thick) and bake at 210° C. for oneminute on a hot plate.

THIRD LAYER

Pattern openings in PiRL layer for stator pole contacts underlying fluxpath layer and contacts to aluminum windings, as well as standoffs forthe rotor.

6. Use the same processing sequence as in step 9 of the previousintegrated stator winding process.

7. Bake the PiRL layer to 300° C. on a hot plate to hard anneal, makingit non-developable in PMMA developer.

8. Sputter 150 Angstroms Ti and 150 Angstroms Ni plating base.

FOURTH LAYER

9. Spin third PiRL layer (about 2 μm) and pattern over the aluminumcontact areas as in step 6 above.

FIFTH LAYER

Deep X-ray lithography layer for stator definition.

10. Use the same processing steps as in steps 11-17 integrated windingprocess.

11. Remove the plating base as in step 18 of the prior integrated statorwinding process.

12. Remove PiRL contact protection layer with 3:1 H₂ O:NH₄ OH.

13. Bond out aluminum current carrying wires using standard gold wirebonding.

SIXTH LAYER

Assemble rotor onto center post. The rotor is fabricated on a separatesubstrate and released via an unpatterned PiRL sacrificial layer whichunderlies the entire rotor structure.

It may be noted that the rotor may have gear teeth defined on itsperimeter as shown above for driving external gears between the statorpoles and, in turn, a mechanical system.

It is understood that the invention is not confirmed to the embodimentsset forth herein as illustrative, but embraces such forms thereof ascome within the scope of the following claims.

What is claimed is:
 1. A method of making a micromachined magneticallyactuated device comprising:(a) forming a sacrificial release layer ofmaterial on the surface of a substrate, the material of the sacrificialrelease layer being dissolvable in a liquid which does not affect thesubstrate; (b) applying a plating base at least over the sacrificialrelease layer, the plating base being formed of a metal; (c) forming alayer of casting material over the plating base to at least the desiredthickness of a rotor structure to be formed, the deposited castingmaterial being susceptible to X-rays such that the casting materialexposed to X-rays can be dissolved in a selected developer solvent; (d)exposing the casting materials to X-rays in a pattern over the platingbase, the pattern including a circular outer ring, a bar-sectionextending through the center of the ring and connecting the ring with noexposure being made of the casting material in openings between the barand the outer ring; (e) removing the casting material with the developersolvent in those areas which have been exposed to X-rays while leavingthe remaining casting material to define a mold area over the platingbase; (f) depositing a solid metal into the mold area onto the platingbase by electroplating to build up a rotor in the mold area; (g)removing the remaining casting material; (h) removing the plating basearound the deposited metal rotor to allow access to the sacrificialrelease layer; (i) removing the sacrificial release layer to free thedeposited metal rotor from the substrate; (j) applying the plating baseonto the surface of another substrate, the plating base being formed ofa metal; (k) forming a layer of casting material over the plating baseto at least the desired thickness of a stator structure to be formed,the deposited casting material being susceptible to X-rays such that thecasting material exposed to X-rays can be dissolved in a selecteddeveloper solvent; (1) exposing the casting material to X-rays in apattern over the plating base, the pattern including areas surrounding acentral region sized to accept the rotor; (m) removing the castingmaterial with the developer solvent in those areas which have beenexposed to X-rays while leaving the remaining casting material to definea mold area over the plating base; (n) depositing a solid metal into themold area onto the plating base by electroplating to form stator polepieces surrounding the region of the rotor, and removing the remainingcasting material to leave the stator pole pieces isolated on thesubstrate; and (o) assembling the rotor into the region between thestator pole pieces.
 2. The method of Claim 1 including during the stepof exposing the casting materials to X-rays in a desired pattern on thesecond substrate, the pattern including a central area in the region ofthe rotor which after removal of the casting material with the developersolvent leaves a central opening, the depositing of solid metal into theopening by electroplating defining a central post, and wherein the rotoris formed with a central opening which accepts the post so that therotor is assembled onto the post to rotate about the post.
 3. The methodof claim 1 wherein the metal deposited to form the rotor and the statorpole pieces is nickel.
 4. The method of claim 1 wherein in the steps ofexposing the casting material to X-rays in a desired pattern, thepattern includes two pairs of pole pieces arranged perpendicularly toone another to provide after electrodepositing the metal on thesubstrate four pole pieces arranged about the rotor.
 5. The method ofclaim 1 wherein the sacrificial release layer is formed of polyimide. 6.The method of claim 1 wherein the step of exposing the casting materialto X-rays is carried out by exposing the casting material to synchrotronradiation.
 7. The method of claim 1 wherein the casting material ispolymethylmethacrylate.
 8. The method of claim 1 wherein the platingbase includes a thin layer of titanium and a thin layer of nickel.